Markets around the world tumbled on Thursday after President Trump announced across-the-board tariffs on America’s main trading partners, including the European Union and Japan.

Futures on the S&P 500, which allow investors to trade the index outside normal trading hours, slumped more than 3 percent. Asian and European stock markets fell sharply, with benchmark indexes dropping more than 3 percent in Japan, and nearly 2 percent in Hong Kong, South Korea, Germany and France.

The value of the U.S. dollar against a basket of other major currencies dropped more than 1 percent.

The slide came after Mr. Trump, speaking at a ceremony at the White House on Wednesday, announced a new 10 percent base line tariff on all imports as well as country-specific taxes on goods from a host of other countries. Those included an additional 34 percent tax on Chinese imports, on top of 20 percent in tariffs he recently put on China, and 20 percent on goods coming from the European Union and 24 percent on Japanese imports.

The market reaction suggested that the scale of the tariffs on Wednesday had come as a surprise, and there was confusion about how the figures had been derived.

“The numbers are shockingly high compared to what people were expecting and it is inexplicable in many ways,” said Peter Tchir, head of macro strategy at Academy Securities. “I think it’s a disaster.”

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